1. Field of the Invention
The present invention relates to a method of detecting undesired signals for cancellation thereof which intermix into a received picture signal in a television signal transmission and hence disturb a signal waveform thereof, for instance, a so-called ghost in a television broadcast, more particularly to the method which is improved so as to extend a range of delay time of the detectable undesired signals such as the ghost, remarkably wider in comparison with the conventional method.
2. Description of the Prior Art
In the ghost cancellation technique which is conventionally effected for removing the ghost components from the received picture signal of the television broadcast, on a sending side, a ghost canceling reference (GCR) signal, which is added to the picture signal in a predetermined horizontal scanning period of a vertical blanking period thereof, is sent, and, on a receiving side, received signal components of the GCR signal itself and ghosts thereof are extracted from the received picture signal and the received ghost components are detected by referring to another GCR signal which is generated on the receiving side, and then the received picture signal is supplied to a ghost canceling filter having a filtering property controlled such that the detected ghost components are canceled, so as to cancel the ghost.
The principle of operation of the aforesaid ghost canceler is based on the employment of a transversal filter which is formed to combine the received picture signals which are successively delayed and the levels of which are adjusted, so as to cancel actual ghost components intermixed into the received picture signal by ghost components which are generated on the receiving side from received signal waveforms of the ghost canceling reference (GCR) signal having the predetermined signal waveform and the predetermined signal level. Accordingly, the GCR signal is added in the midst of a horizontal scanning period in a vertical blanking period, which horizontal scanning period is blanked except regular controlling signals such as a horizontal synchronizing signal, so as to facilitate the aforesaid treatment of ghost components of the GCR signal. However, the detection of those actual ghost components of the GCR signal is disturbed by the superposition thereon of the ghost component of the immediately preceding horizontal synchronizing signal or the superposition thereof on the immediately succeeding horizontal synchronizing signal. As a result, the range of delay time of the detectable and removable actual ghost component is restricted within one half of the horizontal scanning period.
On the other hand, often the ghost component actually disturbing the receipt of the television broadcast is delayed by about 40 .mu.s, which exceeds one half of the horizontal scanning period, e.g. due to reflection from remote high buildings or electrical transmission lines on remote mountains. Consequently, so long as the range of delay time of the detectable ghost component is not exceedingly extended over one half of the horizontal scanning period, the disturbance on the receipt of television broadcast, which is caused by the ghost, cannot be sufficiently prevented, so that the exceeding extension of the range of delay time of the detectable ghost component is a serious problem.